Laser and laser signal combiner

ABSTRACT

An optical communication system for transmitting multiple optical beams, each at a different wavelength is disclosed. The optical communication system includes a laser array having multiple laser transmitters transmitting multiple optical beams, each at a different wavelength. The optical communication system further includes a diffraction grating optically coupled to the laser array, the diffraction grating diffracting each of the optical beams at a substantially equal diffraction angle to form a combined optical beam. The combined beam is then focused into an optical communication media.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/369,492, entitled “LASER AND LASER SIGNAL COMBINER”, filedApr. 1, 2002, the contents of which are incorporated herein by referenceas if set forth in full.

BACKGROUND

Optical communication systems using for example, optical fiber as atransmission medium, have low loss and very high information carryingcapacity. In practice, the bandwidth of optical fiber may be betterutilized to increase the bandwidth of a communication system bytransmitting many distinct channels simultaneously using differentcarrier wavelengths. In WDM (wavelength division multiplexed) networks,for example, multiple optical signals each at different wavelengths arecombined and transmitted over an optical fiber. WDM systems thereforesometimes utilize multiple laser sources that coupled into a singleoptical fiber.

SUMMARY OF THE INVENTION

In one aspect of the present invention an optical communication systemincludes a laser array having a plurality of laser transmitterstransmitting a plurality of optical beams at a plurality of differentwavelengths. The optical communication system further includes adiffraction grating optically coupled to the laser array, thediffraction grating diffracting each of the optical beams at asubstantially equal diffraction angle to form a combined optical beam.The combined beam is then focused into an optical communication media.

In another aspect of the present invention an optical communicationsystem includes a laser array having a plurality of laser transmitterstransmitting a plurality of optical beams at a plurality of differentwavelengths. In this aspect of the invention the optical communicationsystem further includes a virtual image phased array optically coupledto the laser array. In this aspect of the invention the ratio of theangular separation between adjacent lasers in the array of lasersdivided by the wavelength separation between the adjacent lasers in thelaser array is approximately equal to the dispersion of the virtualimage phased array so that the virtual image phased array combines theplurality of optical beams into a combined optical beam. An opticalcommunication media optically coupled to the virtual image phased array,receives the combined optical beam.

In a further aspect of the present invention an optical communicationsystem includes a laser array having a plurality of laser transmitterstransmitting a plurality of optical beams at a plurality of differentwavelengths. In this aspect of the invention the communication systemfurther includes a waveguide grating coupler optically coupled to thelaser array wherein the diffraction order of the grating matches thepropagating mode of the waveguide. An optical communication mediaoptically coupled to the waveguide grating coupler receives the combinedoptical beam.

In a still further aspect of the present invention an opticalcommunication system includes a laser array having a plurality of lasertransmitters formed on a common substrate transmitting a plurality ofoptical beams at a plurality of different wavelengths. In this aspect ofthe invention the communication system further includes an arrayedwaveguide grating monolithically formed on the common substrate, whereinthe arrayed waveguide grating receives the plurality of transmit opticalbeams and combines the plurality of optical beams into a combinedoptical beam. An optical communication media optically coupled to thearrayed waveguide grating receives the combined optical beam.

These and other aspects of the present invention are more readilyunderstood upon viewing the figures indicated below in conjunction withthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of an optical communications systemfor combining a plurality of transmit optical beams transmitted at aplurality of different optical wavelengths by a laser array into asingle fiber;

FIG. 2 illustrates a perspective cross-sectional view of a DFB lasersuitable for use in the laser array of FIG. 1;

FIG. 3 graphically illustrates the relationship between laser frequencyand stripe position of the lasers in the laser array of FIG. 1;

FIG. 4 illustrates another embodiment of an optical communicationssystem for combining a plurality of transmit optical beams transmittedat a plurality of different optical wavelengths by a laser array into asingle fiber;

FIG. 5 illustrates a perspective cross-sectional view of a DFB laserarray with integrated electro-absorption modulators suitable for use inthe laser array of FIG. 1 or FIG. 4;

FIG. 6 illustrates an embodiment of an optical communications system forcombining a plurality of transmit optical beams transmitted at aplurality of different optical wavelengths by a laser array into asingle fiber;

FIG. 7 illustrates another embodiment of an optical communicationssystem for combining a plurality of transmit optical beams transmittedat a plurality of different optical wavelengths by a laser array into asingle fiber; and

FIG. 8 illustrates another embodiment of an optical communicationssystem for combining a plurality of transmit optical beams transmittedat a plurality of different optical wavelengths by a laser array into asingle fiber.

DETAILED DESCRIPTION

An embodiment of the present invention provides an apparatus forcoupling multiple optoelectronic sources into an optical communicationsmedium. For example, FIG. 1 illustrates an optical communication system10 with an array of lasers 20 optically coupled to a collimating lens30. The collimating lens 30 collimates the output optical beam of eachof the lasers in the laser array and forwards the collimated beams 40 toa diffraction grating 50 that diffracts each of the incoming beams atthe same diffraction angle to form a combined optical beam. In thisembodiment a coupling lens 60 collimates the combined optical beam andcouples the collimated combined optical beam into an optical fiber 70.The optical fiber transports the combined optical signal to, forexample, a receiver 90 and/or add/drop multiplexer that demultiplexesthe combined ray in accordance with any of a variety of techniques knownin the art.

In an embodiment of the present invention the array of lasers 20 isformed on a common substrate, with each laser having a different lasingwavelength. In addition, in one embodiment the lasers are, by way ofexample, controlled such that each laser in the laser array can besimultaneously turned on.

In one embodiment the array of lasers 20 are formed from a plurality ofsemiconductor waveguide lasers such as, for example, the ridge waveguidelaser 200 illustrated in FIG. 2. Each of the lasers in the array oflasers may be referred to as a laser stripe. In other embodiments buriedheterostructure, buried rib, or other types of lasers are used.

Typically, the material composition of the waveguide lasers is somecombination of group III-V compound semiconductors, such as GaAs/AlGaAs,InGaAs/AlGaAs or InP/InGaAsP. However, other direct bandgapsemiconductor materials may also be used.

In this embodiment any one of a number of techniques may be used toassign different wavelengths to each laser. For example,directly-written gratings with electron beam lithography, stepping awindow mask during multiple holographic exposures, UV exposure throughan appropriately fabricated phase mask, or changing the effective indexof the mode of the lasers may be used to assign different wavelengths toeach laser.

In some embodiments, a controlled phase shift is also included in thelaser or gain/loss coupling is used in the grating for stable singlemode characteristics. The wavelength of such lasers can be accuratelycontrolled by varying dimensional features, such as stripe width orlayer thickness, across the array.

In one embodiment the lasers may be epitaxially grown on an n-typeindium phosphide (InP) substrate 210. As is conventional in the art, thelaser 200 comprises an un-doped active region 230 disposed between ann-type layer 220 and a p-type layer 240. In an exemplary embodiment, thep-type layer may be doped with suitable dopants known in the art, suchas, for example, zinc (Zn) and the n-type layer may be doped with asuitable dopant such as, for example, silicon (Si).

For example, in one embodiment an n-type InP lower cladding layer 220may be epitaxially formed adjacent the substrate 210. An undoped activeregion 230 comprising at least one small-bandgap InGaAsP quaternaryactive layer sandwiched between a pair of InGaAsP barrier layers (notexplicitly shown) is then formed on the lower cladding layer 220followed by an upper p-type InP cladding layer 240. One of skill in theart will appreciate that the fractional concentrations of In, Ga, As andP may be varied to provide bandgap energy levels as may be preferablefor the formation of the laser diode and low loss optical waveguide.

In this embodiment the upper p-type InP cladding layer 240 is etched inthe shape of a ridge using conventional photolithography. For a DFBlaser, the growth of the device may be interrupted approximately midwaythrough the process and a grating etched into the laser (not shown).After the ridge is etched, the wafer is coated with an insulatingdielectric 250, such as silicon nitride. In this embodiment thedielectric is then removed from on top of the ridge.

A conductive coating or metallization 260 is then applied to the top ofthe ridge. A second metallization step provides a contact region 270,shown at the end of the stripe. In an exemplary embodiment a conductivelayer or metallization 280 may also be deposited on the backside of thesubstrate 210 to form an electrical contact.

In operation, current flowing vertically through the laser, from theupper cladding layer 240 to the substrate contact 280, causes the laserto lase. More specifically, light is generated in the active region 230and confined by the lower cladding layer 220 and the upper claddinglayer 240. The light is guided longitudinally by the ridge structure,which has been etched into the top cladding layer 240. As a result, thelight is confined to oscillate between a partially or anti-reflectingfront facet 290 and a highly reflecting rear facet (not shown).

Returning to FIG. 1, in this embodiment, the stripes in the laser arrayare translated relative to the focal point of the collimating opticallens 30. Therefore, the collimating optical lens 30 presents a differentcompound angle to each of the laser stripes in the laser array.Accordingly, each of the collimated optical beams 40 are incident uponthe diffraction grating 50 at a slightly different angle of incidence inaccordance with the separation between stripes in the laser array.

In this embodiment the focal length of the collimating lens 30, theperiod of the grating, and the positions of the optical elements arechosen such that the diffraction angle is approximately the same foreach beam transmitted by the laser array. Coupling lens 70 then couplesthe diffracted beams into the same optical fiber 80.

For example, diffraction from a periodic grating is governed by thegrating equation given below.mλ/d=sin(θi)+sin(θd)where m is an integer representing the diffraction order, d is thegrating period, λ is the wavelength of the light incident upon thegrating, θi is the angle of incidence, and θd is the diffraction angle.In practice both the angle of incidence and the diffraction angle aremeasured from the normal of the grating surface.

In one embodiment the array of lasers is located at the focal point ofthe collimating lens 30 (see FIG. 1). Thus if x_(n) is the distance fromthe center laser stripe to the nth laser stripe, the grating equationmay written as follows for an array of n lasers.$\frac{m\left( \lambda_{n} \right)}{d} = {{\sin\left( {\theta_{i} + {a\quad{\sin\left( \frac{x_{n}}{F} \right)}}} \right)} + {\sin\quad\theta_{d}}}$where θi is the angle of incidence of the beam transmitted by the centerlaser stripe on the grating, λ_(n) is the wavelength separation betweenchannel and F is the focal length of the collimating lens.

In this embodiment, the plurality of transmit optical beams optimallycouple into the optical fiber when the diffraction angle for each beamincident on the grating is equal. Therefore, assuming that x<<<F optimalcoupling into a single fiber occurs when the difference in wavelength Δλbetween two stripes separated by a distance Δx is given by:${\Delta\lambda} = {\frac{{d \cdot \Delta}\quad x}{m \cdot F} \cdot {\cos\left( \theta_{i} \right)}}$

Therefore, the difference in transmission wavelengths for optimalcoupling decreases with decreasing grating period, d, and increasingangles of incidence θi. For example, in one embodiment the distancebetween adjacent laser stripes is 8 μm and the center wavelength of thelaser array is 1545 nm. In this embodiment the focal length of thecollimating lens is 2 mm so that at normal incidence, the minimumgrating period for which a diffraction order exists is 1545 nm. At thisgrating period, the diffraction angle is 90 degrees, which generally isnot practical. However, the diffraction angle for a grating period of1.6 μm is approximately 75 degrees providing an achievable wavelengthseparation of 6.4 nm.

Therefore, in some embodiments the grating is swept relative to theoptical path of the collimated beams. For example, in one embodiment thecollimated beams are incident on the grating at an angle of incidence ofabout 78.4 degrees. In this embodiment the grating has, by way ofexample, a grating period of 1.0 μm which provides a wavelength spacingi.e. the minimum wavelength separation between adjacent lasers foroptimal coupling of approximately 0.8 nm. The wavelength separation inthis embodiment corresponds to an optical frequency of 100 GHz, atypical channel spacing in DWDM (dense wave division multiplex)networks.

However, for this optical arrangement the relationship between laserfrequency and stripe position is slightly nonlinear, as shown in FIG. 3.Therefore, in an exemplary embodiment the spacing between adjacentlasers in the array of lasers of FIG. 1 is slightly nonlinear to provideaccurate frequency separation.

In some embodiments the grating is, by way of example, placed at therear focal plane of the collimating lens of FIG. 1 to achieve optimumcoupling efficiency. In addition, the diffraction angle for thisembodiment is approximately 34.4 degrees. Therefore it may be difficultto separate the diffracted beams from the forward beams without clippingthe forward beams for this lens-grating spacing and orientation.

Therefore, referring to FIG. 4, in one embodiment a beam splitter 400may be inserted into the optical path to direct the diffracted beam 60out of the forward beam path toward the coupling lens 70. However, theinclusion of the beam splitter may reduce the efficiency of the opticalsystem. Therefore, in other embodiments a transmission, grating ratherthan a reflection grating may be used. In this case, the diffractedbeams are easily accessed however, it may be more difficult to achieve ahigh-efficiency grating.

One of skill in the art will appreciate that the coupling efficiencyinto the optical fiber decreases as the differences between thediffraction angles of the diffracted optical beams increases. Therefore,one embodiment further includes a laser wavelength controller coupled tothe laser array that controls the temperature of the laser array or theexcitation current supplied to the laser transmitters to compensate forloss associated with mismatches that may be present between thediffraction angle associated with the various laser transmitters.

The described exemplary laser combiner may also be utilized to combinethe output from an array of monolithic integrated laser-modulatordevices as illustrated in FIG. 5. In this embodiment, correspondingelectro-absorption modulators 520 a-d are coupled to one or more of theplurality of lasers 510 a-d that form the laser array 500. For example,laser 510 a is coupled to electro-absorption modulator 520 a. In thisembodiment, the electro-absorption modulators modulate the output of thecorreponding laser output.

In one embodiment the optoelectronic device 500 comprises an activeregion 540 disposed between a substrate 530 that also serves as a lowercladding layer and a partially etched upper cladding layer 550 thatforms a ridge type waveguide having an inverted mesa structure. In thisembodiment a lower electrode 560 is disposed below the substrate 530 andseparate upper electrodes (not specifically shown) are formed on theupper surface of the electro-absorption modulators 520 a-d and on theupper surface of the lasers 510 a-d.

In this embodiment each of the lasers are driven by a common drivesignal. In other embodiment the lasers are driven by separate drivesignals for independent device operation using separate drive signallines 570 a-d. Similarly, each of the electro-absorption modulators arealso driven by separate information signals using separate data signallines (e.g. 580).

As previously described with respect to FIG. 1, the spacing betweenstripes in the array of lasers may be on the order of 10 μm or less. Insome embodiments such relative spacing between lasers may result in heatfrom laser stripes affecting operation of adjacent laser stripes.Accordingly, in some embodiments a thermoelectric (TE) cooler (notshown) is thermally coupled to the semiconductor device 500 to controlthe temperature of the lasers. The TE cooler may be abutting or near thesemiconductor device or may be mounted outside of the housing (notshown).

As is known in the art, the TE cooler heats and cools the laser asrequired to drive the output wavelength of the laser to the desiredwavelength. However, the time required to tune the devices with a TEcooler may be relatively long, often on the order of a second. For manyapplications, such as wavelength provisioning in the SONETtelecommunication format, much faster tuning times on the order ofmilliseconds is preferred.

Therefore, one embodiment of the present invention further includesresistive elements coupled to each laser stripe to fine tune thetemperature of individual lasers on a per laser stripe basis. Examplesof structures and methods for tuning on a per laser stripe basis may befound, for example, in U.S. patent application Ser. No. 10/000,141,filed Oct. 30, 2001, entitled LASER THERMAL TUNING, the disclosure ofwhich is incorporated herein by reference as if set forth in full.

An alternative optical communication system 600 having a more compactgeometry is shown in FIG. 6. In this embodiment an optical waveguidegrating coupler 610 combines beams from multiple sources in a laserarray 20 and couples them into an optical transmission media, such as anoptical fiber 670. In this embodiment the array of lasers 20 is againoptically coupled to a collimating lens 30. The collimating lens 30collimates the output optical beam of each of the lasers in the laserarray and forwards the collimated beams 40 to a reflective element 620which forwards reflected beams 630 to a cylindrical lens 640. Thecylindrical lens 640 focuses the reflected beams into the opticalwaveguide grating coupler 610.

In one embodiment the reflective element is, by way of example, a micromirror, such as, for example, a micro-electrical-mechanical structure(MEMS). In this embodiment the micro-mirror fine-tunes the alignmentbetween the array and the grating so that the coupled wavelengthsproperly align with the grating grid.

In one embodiment optical waveguide 650 comprises, for example, anintegrated optical silica waveguide formed on a planar silicon substrate(not explicitly shown). In this embodiment the waveguide is formed bydepositing base, core and cladding layers on a silicon substrate. In oneembodiment the base layer can be made of undoped silica to isolate thefundamental optical mode from the silicon substrate thereby preventingoptical loss at the silica substrate interface.

The core layer is, by way of example, silica doped with an appropriatedopant such as phosphorus or germanium to increase its refractive indexto achieve optical confinement. In one embodiment the cladding comprisesa silica layer that is doped with appropriate dopants such as boronand/or phosphorus to facilitate fabrication and provide an indexmatching that of the base. The waveguide is formed using well-knownphotolithographic techniques.

In this embodiment the grating 660 is, by way of example, composed ofmultiple periodic elements of substantially equal length formed on anupper surface of the optical waveguide. The grating elements intersectthe optical waveguide perpendicularly to its length.

In this embodiment optical beams incident on the grating 660 couple intothe optical waveguide 650 when the diffraction order of the gratingmatches the propagating mode of the waveguide. The conditions forcoupling into the waveguide may be determined as follows:$\beta = {\frac{2 \cdot \pi}{\lambda} \cdot \left( {{n \cdot {\sin(\theta)}} - {i \cdot \frac{\lambda}{\Lambda}}} \right)}$where β is the propagation constant for the waveguide mode, i is thediffracted order of the grating, n is the refractive index of thewaveguide, θ is the angle of incidence, λ is the wavelength of theincident optical beam, and Λ is the grating spacing.

As previously described with respect to FIG. 1, the stripes in the laserarray 20 are offset from the focal point of the collimating optical lens30. Therefore, the collimating optical lens presents a unique compoundangle to each of the laser stripes in the laser array. Accordingly, eachof the collimated optical beams 40 at different optical wavelengths areincident upon the reflective element 620 at a slightly different angleof incidence in accordance with the separation between adjacent stripesin the laser array.

Therefore, the reflective element 620 reflects each of the collimatedoptical beams at a different angle so that each of the beams transmittedby the laser is incident upon the waveguide grating array 620 at adifferent angle of incidence. Therefore, the spacing of the laserstripes in the laser array which operate at predetermined channelspacings may be chosen to provide optimal coupling into the outputoptical fiber.

In this embodiment the output fiber 670 is, by way of example,butt-coupled to the waveguide grating coupler 610. However, in otherembodiments a lens is coupled between the waveguide grating coupler 610and the fiber 670 to focus the combined beam into the fiber. Inaddition, in some embodiments the micro-mirror is used to adjust thealignment between the array and the grating to compensate fortemperature induced wavelength drift in the transmit optical beams aswell as for temperature induced variations in the performance of thewaveguide grating coupler.

Another embodiment of the present invention includes a virtual imagephased array (VIPA) which combines the light from the plurality ofsources into a single fiber. The VIPA has optical properties analogousto a high angular-dispersion grating. (See for example “Large AngularDispersion By A Virtually Imaged Phased Array And Its Application To AWavelength Demultiplexer” by M. Shirasaki, Optics Letters, Vol. 21, No.5, Mar. 1, 1996, the disclosure of which is incorporated herein byreference as if set forth in full).

Referring to FIG. 7, in this embodiment the array of lasers 20 againtransmits a plurality of optical beams, each at a different wavelength.The transmitted optical beams are again optically coupled to thecollimating lens 30. The collimating lens 30 collimates the outputoptical beam of each of the lasers in the laser array and forwards thecollimated beams 40 to VIPA 700.

The VIPA forwards a combined optical waveform including each of theplurality of optical beams to a cylindrical lens 740. The cylindricallens 740 converts the combined waveform to a uniform waveform which isforwarded to a focusing lens 750 that focuses the uniform waveform intoa single optical fiber 760.

The VIPA is formed from a plate 710 made of an optically transparentmaterial, such as, for example, glass, having reflective coatings. 720and 725 formed thereon. In one embodiment reflective coating 725 has, byway of example, a reflectance of approximately 95% or higher, but lessthan 100%. In this embodiment, reflecting coating 720 has a reflectanceof approximately 100%. The VIPA 700 further includes a low reflectivityregion 730 formed on the plate that is optically coupled to thecollimating lens to receive the collimated optical beams.

In this embodiment the VIPA 700 outputs optical beams at an output anglethat varies as a function of the wavelength of the input light and angleof incidence. For example, when input light 40 is at a wavelength λ₁ andincidence angle θ, VIPA 700 outputs an optical beam at wavelength λ₁ ina specific angular direction. Similarly, when input light 40 is at awavelength λ₂ with the same incidence angle, VIPA 700 outputs an opticalbeam at wavelength λ₂ in a different angular direction. Therefore, ifthe laser array transmits optical beams at wavelengths λ₁-λ_(n) whichare incident on the VIPA at the same incident angle, the VIPAsimultaneously outputs separate optical beams at wavelengths λ₁-λ_(n) indifferent directions.

However, in this embodiment the stripes in the laser array 20 are againtranslated relative to the focal point of the collimating optical lens30. Therefore, the collimating optical lens presents a unique compoundangle to each of the laser stripes in the laser array. Accordingly, eachof the collimated optical beams 40 at unique frequencies are incidentupon the VIPA 700 at a slightly different angle of incidence as afunction of the separation between laser stripes.

In this embodiment the plurality of optical beams transmitted by thelaser array optimally couple into the optical fiber when the ratio ofthe angular separation between adjacent stripes divided by thewavelength separation between adjacent stripes is approximately equal tothe dispersion (i.e. angular change in output signal divided by thechange in wavelength) of the VIPA. For example, the dispersion for a0.100 mm thick glass plate installed at a small angle of incidence tothe incoming optical beam is on the order of 1 deg/nm. Therefore, inthis embodiment the transmit optical beams will optimally coupled intothe optical fiber if the dispersion (i.e. the ratio of the spacing andwavelength separation between adjacent stripes) is equal toapproximately 1 deg/nm.

In one embodiment the DFB array has a spacing, Δx, of 8 μm betweenelements. The array in this embodiment is, by way of example, located atthe focal point of a collimating lens having a 1 mm focal length, F.Therefore, the angular separation of the elements (i.e. sin⁻¹(Δx/f)) inthis embodiment is approximately 0.5 deg. Therefore, in this embodimentthe wavelength separation between adjacent sources is on the order of0.5 nm to match the dispersion of the VIPA to optimally couple thetransmit optical beams into the output optical fiber.

In one embodiment the VIPA 700 is, by way of example, rotated tooptimally align the DFB laser array 20 with the output fiber.Alternatively a MEMs mirror may be optically coupled between thecollimating lens 20 and VIPA 700 or between the cylindrical lens 740 andthe fiber coupling lens 750 to provide the necessary alignment. The MEMsmirror may also adjust the alignment to compensate for temperatureinduced variation in the system.

In an alternate embodiment a dispersive component such as an Eschellegrating or an arrayed waveguide grating (AWG) is integrated with thelaser array to provide a monolithic solution. The monolithic solutionavoids the loss that may be associated with an integrated opticalcombiner. The AWG for example, has low loss, does not need exactingvertically etched mirrors and is readily integrated with activeelements.

Referring to FIG. 8, in another embodiment an arrayed waveguide grating(AWG) couples the outputs from the plurality of optical sources in theDFB laser array 20 into a common optical fiber. In this embodiment theAWG includes an input waveguide 800 coupled to the laser array 20 toreceive the transmitted optical beams. The input waveguide 800 iscoupled with a first waveguide coupler 810 which in turn is coupled withan array of waveguides 820. The array of waveguides 820 terminate in asecond waveguide coupler 830 which is coupled to an output optical fiber840.

In an exemplary embodiment the length of the individual waveguides andshape of the couplers are chosen so that input beams of predeterminedwavelengths pass through the array of waveguides and create adiffraction pattern on the output optical fiber 840. The AWG thereforecouples the plurality of input beams into a single output. (see U.S.patent application Ser. No. 10/000,142, filed Oct. 30, 2001, entitledTUNABLE CONTROLLED LASER ARRAY, the disclosure of which is incorporatedby reference).

Arrayed waveguide gratings may be used as demultiplexers in thereceiving end of a WDM link. These AWGs may be fabricated insilica-on-silicon. However, AWGs formed from InP waveguides may also beused as dispersive components monolithically integrated with an array ofInP detectors to form a multi-channel receiver. Similarly, AWGs may beintegrated into the laser cavity of an optoelectronic source to providefeedback to determine the lasing wavelength. However, the longitudinalmodes of such devices are unstable due to the long cavity length.

Similarly, dispersive components such as PHASARS or AWGs may also beintegrated with with an array of distributed Bragg relfectors (DBRs)lasers. However, in these devices the DBRs are only tunable over anarrow frequency range and the AWG only works to combine the output ofall the lasers into a single waveguide, and is not part of the lasingcavity.

In the present invention the DFB laser array includes quarter-wave phaseshifts to provide high modal stability. Thus all the lasers stay singlemode with high side-mode-suppression-ratios. In addition, in thedescribed exemplary embodiment the dependence of the wavelength of theoptical beams transmitted by the laser array is approximately equal tothe wavelength dependence of the AWG. Therefore, as the temperature ofthe chip is changed, the wavelength of the DFB array and the AWG shifttogether maintaining alignment between the DF array and AWG as well asthe tuning of the grating.

In one embodiment the drive current of individual lasers is varied tofine tune the output wavelength of individual device so that the entirearray is properly registered in wavelength to the AWG. Varying the drivecurrent of individual devices has the parasitic effect of changing thepower emitted by each laser. However, the mismatch in registrationbetween the wavelengths of the AWG and the laser array generally has amuch larger effect on the total output power. In fact, equal power canoften be obtained from all channels if the current to each laser isproperly adjusted, since the sharp wavelength characteristics of the AWGmoderate any increase in laser power obtained by increasing the lasingcurrent.

Additionally in one embodiment resistive elements are coupled to eachlaser stripe to fine tune the temperature and wavelength of individuallasers on a per laser stripe basis as described with respect to FIG. 5.Examples of structures and methods for tuning on a per laser stripebasis may be found, for example, in U.S. patent application Ser. No.10/000,141, filed Oct. 30, 2001, entitled LASER THERMAL TUNING,integrated heaters can also be used to fine tune the wavelength of eachlaser.

In this embodiment the AWG and the DFB array are monolithically formedon a single chip. Therefore, in this embodiment, the DFB array and AWGare automatically aligned by the lithographic process used to fabricatethe chip. For example, in one embodiment the active waveguide is formedover the entire wafer. However, the heavy doping typically used in thep-type cladding of many lasers would absorb too much light in the AWG.Similary, the quantum wells in the laser active region would present toomuch loss in the AWG structure. Therefore, in some embodiments the uppercladding and active region comprising one or more quantum wells in thepassive region of the chip occupied by the AWG are removed by a standardetching process.

In this embodiment a non-absorbing waveguide and an undoped uppercladding are then re-grown on this passive region of the chip. In thisembodiment the AWG waveguide contains a non-absorbing waveguide, wherethe bandgap of the core region is sufficiently higher than the operatingwavelengths to present negligible absorption. In addition, the waveguidestructure of both the AWG and the laser are, by way of example, formedtogether in a single mask step, using a ridge or a buried process.

An alternate embodiment uses a continuous waveguide under the activeregion in lieu of the direct coupled method described above.Alternatively, two vertically coupled waveguides with transition regionsbetween the active and the passive sections are used in someembodiments.

Although this invention has been described in certain specificembodiments, many additional modifications and variations would beapparent to one skilled in the art. It is therefore to be understoodthat this invention may be practiced otherwise than is specificallydescribed. Thus, the present embodiments of the invention should beconsidered in all respects as illustrative and not restrictive. Thescope of the invention to be indicated by the appended claims, theirequivalents, and claims supported by the specification rather than theforegoing description.

1. An optical communication system, comprising: a laser array having aplurality of laser transmitters transmitting a plurality of opticalbeams at a plurality of different wavelengths; a diffraction gratingoptically coupled to said laser array, the diffraction gratingdiffracting each of the optical beams at a substantially equaldiffraction angle to form a combined optical beam; and an opticalcommunication media optically coupled to the diffraction grating, theoptical communication media receiving the combined optical beam.
 2. Theoptical communication system of claim 1 wherein each of the plurality ofoptical beams transmitted by the laser array is incident upon thediffraction grating at a different angle of incidence.
 3. The opticalcommunication system of claim 2 wherein the angle of incidence on thegrating for each of the plurality of optical beams transmitted by thelaser array varies in accordance with separation in wavelength betweenoptical beams formed by adjacent laser transmitters.
 4. The opticalcommunication system of claim 1 further comprising a collimating lensoptically coupled between the laser array and diffraction grating,wherein the collimating lens collimates the transmit optical beams andforwards collimated beams to the diffraction grating.
 5. The opticalcommunication system of claim 4 further comprising a focusing lensoptically coupled between the diffraction grating and the opticalcommunication media wherein the focusing lens focuses the combinedoptical beam into the optical communication media.
 6. The opticalcommunication system of claim 1 wherein spacing in wavelength betweenoptical beams formed by adjacent lasers in the laser array is nonlinear.7. The optical communication system of claim 1 further comprising areceiver coupled to the optical communication media.
 8. The opticalcommunication system of claim 1 further comprising one or moreelectro-absorption modulators monolithically integrated with one or moreof the plurality of laser transmitter in the laser array, wherein theelectro-absorption modulators modulate the optical beam of acorresponding laser transmitter in accordance with an informationsignal.
 9. The optical communication system of claim 1 wherein thediffraction grating comprises a reflection diffraction grating.
 10. Theoptical communication system of claim 1 wherein the diffraction gratingcomprises a transmission diffraction grating.
 11. The opticalcommunication system of claim 4 further comprising a beam splitteroptically coupled between the collimating lens and the diffractiongrating, wherein the beam splitter separates the collimated opticalbeams from the diffracted optical beams. 12.-19. (canceled)
 20. Anoptical communication system comprising: a laser array having aplurality of laser transmitters transmitting a plurality of opticalbeams at a plurality of different wavelengths; a waveguide gratingcoupler optically coupled to said laser array wherein a diffractionorder of the grating matches a propagating mode of the waveguide; and anoptical communication media optically coupled to the waveguide gratingcoupler, wherein the optical communication media receives the combinedoptical beam.
 21. The optical communication system of claim 20 whereineach of the plurality of optical beams transmitted by the laser array isincident upon the waveguide grating coupler at a different angle ofincidence.
 22. The optical communication system of claim 21 wherein theangle of incidence on the waveguide grating coupler for each of theplurality of optical beams transmitted by the laser array varies inaccordance with separation in wavelength between optical beams formed byadjacent laser transmitters.
 23. The optical communication system ofclaim 20 further comprising a collimating lens optically coupled to thelaser array, wherein the collimating lens collimates the plurality oftransmitted optical beams and forwards the collimated optical beams to amicro-mirror that aligns the collimated optical beams with the waveguidegrating.
 24. An optical communication system comprising: a laser arrayhaving a plurality of laser transmitters formed on a common substratetransmitting a plurality of optical beams at a plurality of differentwavelengths; an arrayed waveguide grating monolithically formed on thecommon substrate, wherein the arrayed waveguide grating receives theplurality of transmit optical beams and combines the plurality ofoptical beams into a combined optical beam; and an optical communicationmedia optically coupled to the arrayed waveguide grating, wherein theoptical communication media receives the combined optical beam.
 25. Theoptical communication system of claim 24 wherein length of individualwaveguides in the arrayed waveguide grating are such that the pluralityof optical beams are coupled to the optical communication media.